No pictures with this entry...
So an overseas work trip (I tend to have a lot of those in my line of work) put a temporary halt to progress, however it was not completely wasted time as I did manage to get some online shopping done... most of the internals of the amplifier (resistors, capacitors, rectifier diodes, valve sockets etc) were ordered online and are on their way.
The peer review of the circuit design was in equal parts useful and discouraging; a thin veneer of useful pointers and advice, wrapped around a solid core of "you don't know what you're doing, go away and leave us alone". Yeah, definitely outstayed my welcome on that one :(
I've also decided that I'm going to build up an experimental/test version of the gain, splitter and driver stages of the amp in a small test chassis with wide spacing between tagboards, to facilitate experimentation and component substitution. I want to DC-couple the anode of the initial gain stage to the grid of the cathodyne/concertina phase inverter, Williamson-style. Using a 12AX7 this should be possible, but the calculations are marginal.
So, the plan is to build up the input stages on a test chassis, using a function generator for input, and monitoring the voltages and waveforms at critical points in the circuit using the voltmeter and oscilloscope. Of course with no output stage this won't be anything that can be connected to a loudspeaker or headphone and listened to, but the voltmeter and oscilloscope will tell the story.
If successful, I shall post conspicuously on this blog the operating parameters necessary to use a 12AX7 in this configuration, with oscilloscope traces etc – in the hopes it may help someone else.
And if it is not possible to get an effective result, I'll switch to using the 6SN7, but I want to try with the 12AX7 / 12AU7 valves first.
When the components start arriving, I'll start building...